The Institute of Electrical and Electronic Engineers (IEEE) manages most of the worldwide standards for computer local area networks (LANs) and its IEEE 802.11 Standard represents the first standard for wireless LAN (WLAN) products. The majority of the WLAN products available in the marketplace today are proprietary spread spectrum solutions targeting vertical applications operating in the 900 MHz and 2.4 GHz frequency bands. These products include wireless adapters and access points in PCMCIA, ISA and custom PC board platforms. Proprietary solutions for some applications are beneficial, especially for those requiring market differentiation or customization of a wireless LAN network. However, proprietary solutions are typically customized and constrain end users into purchasing products from a single equipment supplier. The advantage of standards-based products is that users can choose from a number of vendors that provide compatible products. This increases competition and provides the potential for lower cost products. Thus, interoperability, low cost and stimulation of market demand are some of the advantages that standards-based solutions offer.
The IEEE 802.11 standard defines the protocol for two types of networks: Ad-hoc and client/server networks. An Ad-hoc network is a simple network where communications are established between multiple stations in a given coverage area without the use of an access point or server. The standard specifies the etiquette or protocol that each station must observe so that they all have fair access to the wireless communication link. It also provides schemes for arbitrating requests to use the communication link to ensure that throughput is maximized for all users. In contrast, client/server networks use an access point that controls the allocation of bandwidth (i.e., transmission times) for all stations. The access point may also be used to handle traffic to and from a wired or wireless backbone. This arrangement allows for point coordination of all of the stations in the network and ensures proper handling of the data traffic as the access point routes data between the stations and to and from the network. Typically WLANs controlled by a central access point will provide better throughput performance.
The IEEE 802.11 standard does not specify technology or implementation but simply provides specifications for the physical layer and Media Access Control (MAC) layer of a network. The standard thus allows for manufacturers of wireless LAN radio equipment to build interoperable network equipment, while still providing design freedom for these vendors to choose desired implementations.
The physical layer (or phy) in any network defines the modulation and signaling characteristics for the transmission of data. At the physical layer of an 802.11-compliant network, two RF (radio frequency) and one infrared transmission methods are defined. For purposes of this discussion, only the RF methods are considered, but in general the schemes described herein may be equally applicable to infrared transmissions. Operation of a WLAN in unlicensed RF bands requires the use of spread spectrum modulation to meet the requirements for operation in most countries. The RF transmission standards set forth in the 802.11 standard are Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). Both architectures are defined for operation in the 2.4 GHz frequency band, typically occupying the 83 MHz of bandwidth from 2.400 GHz to 2.483 GHz. Differential band phase shift keying (DBPSK) and differential quadrature phase shift keying (DQPSK) are the modulation techniques employed for the DSSS networks. Frequency hopping networks use 2–4 level Gaussian FSK (frequency shift keying) as the modulation signaling method. The physical layer data rate for FHSS systems is 1 Mbps. For DSSS systems both 1 Mbps and 2 Mbps data rates are supported. The choice between FHSS and DSSS will depend on a number of factors related to the users application and the environment that the system will be operating within.
The DSSS physical layer uses an 11-bit Barker code Sequence to spread the data before it is transmitted. Each bit transmitted is modulated by the 11-bit sequence. This process spreads the RF energy across a wider bandwidth than would be otherwise required to transmit the raw data. The processing gain of the system is defined as 10x the log of the ratio of spreading rate (also know as the chip rate) to the data. The receiver “despreads” the received signal to recover the original data. The advantage of this technique is that it reduces the effect of narrowband sources of interference. The spreading architecture used in the direct sequence physical layer should not be confused with code division multiple access (CDMA) networks. All 802.11 compliant products utilize the same pseudorandom (PN) code and therefore do not have a set of codes available as is required for CDMA operation.
The FHSS physical layer has 22 hop patterns to choose from. The frequency hop physical layer is required to hop across the 2.4 GHz band covering 79 channels. Each channel occupies 1 Mhz of bandwidth and transmitters must hop at a specified minimum rate (2.5 hops per second in the United States). Each of the physical layers use their own unique header to synchronize with the receiver and to determine the signal modulation format and data packet length. The physical layer headers are always transmitted at 1 Mbps. Predefined fields in the headers provide the option to increase the data rate to 2 Mbps for the actual data packet.
The MAC layer specification for the 802.11 standard has similarities to the 802.3 Ethernet wired line standard. 802.11 networks use a protocol scheme know as carrier-sense, multiple access, collision avoidance (CSMA/CA). This protocol seeks to avoid data collisions instead of detecting them such as the algorithm used in the 802.3 standard. It is difficult to detect collisions in an RF transmission network and it is for this reason that collision avoidance is used.
The MAC layer operates together with the physical layer by sampling the energy over the medium transmitting data. The physical layer uses a clear channel assessment (CCA) algorithm to determine if the channel is clear. This is accomplished by measuring the RF energy at the antenna and determining the strength of the received signal. This measured signal is commonly known as RSSI. If the received signal strength is below a specified threshold the channel is declared clear and the MAC layer is given the clear channel status for data transmission. If the RF energy is above the threshold, data transmissions are deferred in accordance with the protocol rules.
The standard provides another option for CCA that can be used in place of or together RSSI measurement. Carrier sense can be used to determine if the channel is available. This technique is more selective because it verifies that the signal is the same carrier type as 802.11 transmitters. The best method to use depends upon the levels of interference in the operating environment.
The CSMA/CA protocol allows for options the can minimize collisions by using request to send (RTS), clear-to-send (CTS), data and acknowledge (ACK) transmission frames, in a sequential fashion. Communications are established when one of the wireless nodes sends a short message RTS frame. The RTS frame includes the destination and the length of message. The message duration is known as the network allocation vector (NAV). The NAV alerts all others in the network, to back off the communication link for the duration of the transmission. The receiving station issues a CTS frame which echoes the senders address and the NAV. If the CTS frame is not received, it is assumed that a collision occurred and the RTS process starts over. After the data frame is received, an ACK frame is sent back verifying a successful data transmission.
A common limitation with wireless LAN systems is the “hidden node” problem. This can disrupt a significant volume of the communication traffic in a highly loaded LAN environment. It occurs when there is a station in a service set that cannot detect the transmission of another station to detect that the media is busy. In such cases two or more stations that are unaware of one another's transmissions may try to transmit at the same time to another station that can “hear” more than one of the transmitting stations. The use of RTS, CTS, Data and ACK sequences helps prevent the disruptions caused by this problem.
Thus, the 802.11 standard defines practices that ensure network interoperability, however, as indicated above, the standard does not specify any particular implementation details. For example, the 802.11 standard makes no mention of how protocol functions should be divided between hardware and software operations in an 802.11 compliant product. Such decisions are left to product vendors.